The present invention generally relates to guide drums which comprise a stationary drum and a rotary drum and are for use in so-called helical scan type magnetic recording and/or reproducing apparatuses which record and/or reproduce a signal on and/or from a magnetic tape, and more particularly to a guide drum which is designed so that the magnetic tape can move smoothly and a satisfactory recording and/or reproduction can be carried out.
Generally, when recording or reproducing a signal on or from a magnetic tape in a helical scan type magnetic recording and/or reproducing apparatus (hereinafter simply referred to as a video tape recorder or VTR), the tape is moved in a state where the tape is wrapped spirally around an outer peripheral surface of a guide drum over a predetermined angular range. The guide drum comprises a substantially cylindrical stationary drum and a substantially cylindrical rotary drum. The stationary drum is provided with a tape guide for guiding the tape along a predetermined tape path. The rotary drum is provided with one or a plurality of magnetic heads which project slightly from an outer peripheral surface of the rotary drum. At an entrance part of the guide drum in a tape moving direction, a large portion of the tape along a width direction thereof is in contact with the outer peripheral surface of the rotary drum. On the other hand, at an exit part of the guide drum in the tape moving direction, a large portion of the tape along the width direction thereof is in contact with an outer peripheral surface of the stationary drum. The rotary drum rotates at a high speed of 1800 rpm, for example, and the head scans obliquely to a longitudinal direction of the tape so as to record or reproduce the signal on or from the tape.
Because the rotary drum rotates at the high speed, an air flow is generated in a vicinity of the outer peripheral surface of the guide drum and an air film is formed by this air flow between the guide drum and the tape. A thickness of the air film is not uniform throughout the entire angular range over which the tape is wrapped around the outer peripheral surface of the guide drum. The thickness of the air film is largest at the entrance part of the guide drum along the tape moving direction and gradually decreases toward the exit part of the guide drum, since a large area of the tape opposes the outer peripheral surface of the rotary drum at the entrance part of the guide drum and only a small area of the tape opposes the exit part of the guide drum. Due to the formation of such a non-uniform air film, the contact between the head and the tape becomes unstable, and problems such as a shew in a reproduced picture occurs because lengths of tracks formed on the tape become inconsistent.
Hence, in a conventional VTR, a diameter of the outer peripheral surface of the rotary drum opposing the tape is set greater than a diameter of an outer peripheral surface of the stationary drum opposing the tape. As a result, the head makes positive contact with the tape. However, at the exit part of the guide drum along the tape moving direction, the tape makes contact with the outer peripheral surface of the rotary drum and the tape is easily damaged. In addition, particles of magnetic material scraped off from a recording surface of the tape due to the contact between the tape and the outer peripheral surface of the rotary drum adhere to the tape and the head, and there is a problem in that a signal dropout may occur as a consequence. Further, an unstable rotation of the rotary drum due to vibration and eccentricity of the rotary drum is directly transmitted to the tape and causes problems such a wow and flutter and jitter.
When the tape makes contact with the outer peripheral surface of the rotary drum at the exit part of the guide drum along the tape moving direction or when the thickness of the air film is not uniform from the entrance part toward the exit part of the guide drum, a linearity of tracks formed on the tape becomes poor. Moreover, a load on a drum motor which drives the rotary drum increases when the tape makes contact with the outer peripheral surface of the rotary drum, and there is a problem in that a power consumption becomes large. On the other hand, depending on a projecting quantity of the head from the outer peripheral surface of the rotary drum, a hitting phenomenon occurs. In the present specification, the hitting phenomenon refers to a phenomenon in which the head hits the recording surface of the tape and separates from the tape at the exit part of the guide drum. This hitting phenomenon is especially notable when a distance between the recording surface of the tape and the outer peripheral surface of the rotary drum at the exit part of the guide drum is small compared to the projecting quantity of the head from the outer peripheral surface of the rotary drum. When the hitting phenomenon occurs, the recording surface of the tape is hit by the head and is easily damaged, and a vibration easily occurs when the head hits the tape.
It is possible to eliminate the hitting phenomenon by reducing the projecting quantity of the head from the outer peripheral surface of the rotary drum. However, in this case, there is a problem in that it no longer becomes possible to maintain positive contact between the head and the tape at the entrance part of the guide drum along the tape moving direction.
On the other hand, recently, metal tapes and metal evaporated thin film tapes such as metal alloy thin film tapes are also used on VTRs. However, these kinds of tapes are more rigid compared to the normal tape, and it is extremely difficult to stably maintain a desired contact pressure of the head with respect to the tape by use of the conventional guide drum. For this reason, it is impossible to carry out a satisfactory recording or reproduction by bringing out the fine performance of the tape.
Accordingly, in order to eliminate the problems described heretofore, a guide drum having a tapered part was previously proposed in a Japanese Laid-Open Utility Model Application No. 58-148757 which was laid open on Oct. 6, 1983. The previously proposed guide drum comprises a rotary drum and a stationary drum, wherein the rotary drum is provided with the tapered part at a lowermost part thereof. An outer diameter of the tapered part decreases toward the stationary drum. According to this previously proposed guide drum, the tape does not make contact with the rotary drum at the exit part of the guide drum along the tape moving direction due to the provision of the tapered part, and the hitting phenomenon does not occur. However, the proposed guide drum does not consider controlling the thickness of the air film at the entrance part of the guide drum along the tape moving direction. As a result, a difference between the thickness of the air film at the entrance part and the thickness of the air film at the exit part becomes large. Consequently, the tape tension becomes unstable and the tape movement also becomes unstable. Moreover, since the contact pressure of the head with respect to the tape becomes unstable, it becomes impossible to carry out a satisfactory recording or reproduction.